1. Field of the Invention
The present invention relates to a tracking servo control for enabling a radiation position of a light beam to follow a track of a recording disk, and more particularly relates to a tracking error signal compensation circuit and a tracking error signal compensation method for removing an offset of a tracking error signal used for a tracking servo control.
2. Description of the Related Art
The tracking servo control is carried out when reproducing a recording disk, such as an optical disk, a magneto-optical disk or the like. The tracking servo control is the control for enabling a radiation position of a light beam to follow a track formed on the recording disk.
A tracking servo circuit is mounted in a reproducing apparatus or a recording apparatus of the recording disk. The tracking servo circuit uses a reflection light generated by the radiation of the light beam to the recording disk to thereby generate a tracking error signal. The tracking error signal is a signal in which an amplitude is changed on the basis of a deviation between a track position and the radiation position of the light beam. The amplitude of the tracking error signal is at a zero level when the radiation position of the light beam is located directly over the track. Thus, the tracking servo circuit controls the radiation position of the light beam so that the amplitude of the tracking error signal becomes at the zero level. As a result, the radiation position of the light beam follows the track.
However, when the offset is included in the tracking error signal, a deviation corresponding to the offset is induced between the track position and the radiation position of the light beam, even if the radiation position of the light beam is controlled such that the amplitude of the tracking error signal is at the zero level. This results in an inaccurate operation of the tracking servo control. This offset is caused by the variation of performances of respective elements constituting the tracking servo circuit or the optical pickup and the like, in many cases.
For this reason, the tracking error signal compensation circuit is mounted in the tracking servo circuit. The tracking error signal compensation circuit is also referred to as a tracking balance circuit. This circuit is intended to remove the offset in the tracking error signal.
The tracking error signal compensation circuit is provided with: a low pass filter (LPF) for extracting a direct current component, namely, an offset component from the tracking error signal; a comparator for comparing the offset component with a reference value and then outputting a difference between them; and an adder (or a subtracter) for adding the difference on the tracking error signal (or subtracting the difference from the tracking error signal).
A process of removing the offset from the tracking error signal is carried out before reproducing information recorded on the recording disk, for example, immediately after the recording disk is installed in the reproducing apparatus. Immediately after the installation of the recording disk, the reproducing apparatus rotates the recording disk and radiates the light beam to the recording disk, in a condition that a feedback loop of the tracking servo control is opened. At this time, if the recording disk is eccentric, the recording disk is reciprocated in a radius direction thereof with respect to the fixed radiation position of the light beam. Accordingly, the light beam strides the tracks formed on the recording disk many times. The amplitude of the tracking error signal is changed into a substantially sine wave form, while the radiation position of the light beam is shifted from a position directly over the track to a position directly over the adjacent track. For this reason, the tracking error signal of a successively sine wave form can be generated if the light beam strides the tracks formed on the recording disk many times. The tracking error signal compensation circuit passes this tracking error signal through the low pass filter to thereby extract the direct current component thereof, namely, the offset component. Then, the difference between the offset component and the reference value is generated by the comparator. This difference is added to (or subtracted from) the tracking error signal. Accordingly, the offset of the tracking error signal is removed.
As mentioned above, if the recording disk is eccentric, the recording disk is reciprocated in the radius direction with respect to the fixed radiation position of the light beam. So, if the reciprocation of the recording disk is observed with the fixed radiation position of the light beam as a standard point, a moving speed of the recording disk is the slowest when a moving direction of the recording disk is switched to the reverse direction at an end of the reciprocation. On the other hand, the moving speed of the recording disk is relatively fast while the recording disk is moved from one end of the reciprocation to the other end.
A frequency of the tracking error signal is changed in response to such a change of the speed of the reciprocation of the recording disk. Thus, the frequency of the tracking error signal is the lowest when the moving direction of the recording disk is switched to the reverse direction.
In order to extract the offset component in the tracking error signal by using the low pass filter mounted in the tracking error signal compensation circuit, it is necessary to make a cutoff frequency of the low pass filter sufficiently lower than the lowest frequency of the tracking error signal. If the cutoff frequency of the low pass filter is made lower, a time constant is made larger. Thus, a time until an output of the low pass filter is stable is made longer. This results in a problem that a time of the process of removing the offset from the tracking error signal is long.
On the other hand, as mentioned above, the process of removing the offset from the tracking error signal is carried out before reproducing the information recorded on the recording disk, for example, immediately after the recording disk is installed in the reproducing apparatus. However, even if the offset is removed from the tracking error signal immediately after the installation of the recording disk, this process has a problem that the offset may be induced in the tracking error signal during the reproduction of the recording disk. In addition, the recording apparatus of the recording disk has the problems similar to those of the reproducing apparatus.
It is therefore a first object of the present invention to provide a tracking error signal compensation circuit and a tracking error signal compensation method which can shorten the time of the process of removing the offset from the tracking error signal.
A second object of the present invention is to provide a tracking error signal compensation circuit and a tracking error signal compensation method which can remove the offset in the tracking error signal induced during the reproducing operation or the recording operation.
A tracking error signal compensation device in accordance with the present invention is a device or a circuit for removing an offset of a tracking error signal to be used for a tracking servo control for causing a radiation position of a light beam radiated by an optical pickup to follow to a track of a recording disk.
The tracking error signal compensation device includes: a low pass filter that extracts a direct current component and a low frequency component whose frequency is lower than a reference frequency from the tracking error signal, and generates a detection signal including the extracted direct current component and the extracted low frequency component; a determination device that determines whether or not a frequency of the tracking error signal is lower than a reference frequency; a holding device that generates a hold signal by holding the detection signal generated by the low pass filter, while the determination device determines that the frequency of the tracking error signal is lower than the reference signal; a detection device that detects the offset of the tracking error signal by using the detection signal and the hold signal; and a removing device that removes the offset detected by the detection device from the tracking error signal.
The tracking error signal is obtained from the optical pickup. The obtained tracking error signal is supplied to the tracking error signal compensation device. In the tracking error signal compensation device, the tracking error signal is supplied to the low pass filter and the determination device. The low pass filter extracts a direct current component and a low frequency component whose frequency is lower than a reference frequency from the tracking error signal, and generates a detection signal including the extracted direct current component and the extracted low frequency component. The detection signal is supplied to the holding device. The determination device determines whether or not a frequency of the tracking error signal is lower than a reference frequency. The holding device generates a hold signal by holding the detection signal, while the determination device determines that the frequency of the tracking error signal is lower than the reference signal. The detection signal and the hold signal are supplied to the detection device. The detection device detects the offset of the tracking error signal by using the detection signal and the hold signal. The removing device removes the offset detected by the detection device from the tracking error signal.
The detection of the offset of the tracking error signal needs to extract the direct current component from the tracking error signal. The extraction of the direct current component can be done with the low pass filter. However, as mentioned above, the frequency of the tracking error signal is partly low. That is to say, the tracking error signal contains a low frequency component.
If only the direct current component is perfectly extracted from the tracking error signal with a low pass filter, it is needed to use a low pass filter whose cutoff frequency is lower enough to cut the low frequency component. However, the use of such a low pass filter makes the operation of the tracking error signal compensation device slow, because the time constant of the low pass filter is large.
To solve this problem, the low pass filter of the tracking error signal compensation device in accordance with the present invention allows to pass not only the direct current component in the tracking error signal but also the low frequency component in the tracking error signal whose frequency is lower than the reference frequency. This means that the cutoff frequency of this low pass filter is relatively high, so that the time constant of the low pass filter is relatively small. Therefore, the low pass filter operates quickly.
The low pass filter of the tracking error signal compensation device in accordance with the present invention outputs the detection signal containing the direct current component and the low frequency component. Therefore, if the detection of the offset of the tracking error signal is carried out by using of this detection signal, it is needed to remove the low frequency component from the detection signal.
The removal of the low frequency component from the detection signal is done by the determination device that determines the frequency of the tracking error signal and the holding device that generates the hold signal. The determination device determines whether or not the frequency of the tracking error signal is lower than the reference frequency. On the basis of determination of the determination device, it is possible to recognize a location or time that the low frequency component appears in the detection signal. On the basis of the determination of the determination device, the holding device holds the detection signal. Thus, the hold signal representing an artificial direct current component of the tracking error signal is generated, and the low frequency component in the detection signal is replaced with the hold signal. Accordingly, the low frequency component is removed from the detection signal.
In this manner, the direct current component of the tracking error signal is substantially perfectly extracted, so that the detection of the offset of the tracking error signal and the removal of the offset from the tracking error signal can be done.
Accordingly, the quick operation of the low pass filter and the removal of the offset from the tracking error signal can be simultaneously achieved. Thus, a tracking error signal compensation circuit which can shorten the time of the process of removing the offset from the tracking error signal can be provided.
A tracking error signal compensation device in accordance with another aspect of the present invention includes: a track jump detection device that detects a track jump of the optical pickup; a detection device that detects the offset of the tracking error signal; and a removing device that removes the offset detected by the detection device from the tracking error signal. In this tracking error signal compensation device, the detection device and the removing device are carried out removal of the offset from the tracking error signal, when the track jump detection device detects the track jump of the optical pickup.
The tracking error signal can be obtained from the optical pickup at the time of the track jump. For example, the optical pickup is moved in the radial direction of the recording disk at the time of the track jump. If the optical pickup continues to radiate the light beam to the recording while the optical pickup is moved in the radial direction of the recording disk, the tracking error signal is thus obtained, because the light beam strides tracks.
The tracking error signal compensation device in accordance with the present invention carries out the removal of the offset from the tracking error signal by using the tracking error signal obtained at the time of track jump. Thus, a tracking error signal compensation circuit which can remove the offset in the tracking error signal induced during the reproducing operation or the recording operation can be provided.
A tracking error signal compensation method in accordance with the present invention is a method of removing an offset of the tracking error signal. The method includes the processes of: extracting a direct current component and a low frequency component whose frequency is lower than a reference frequency from the tracking error signal by a low pass filter, and generating a detection signal including the extracted direct current component and the extracted low frequency component; determining whether or not a frequency of the tracking error signal is lower than a reference frequency; generating a hold signal by holding the detection signal generated by the low pass filter, while it is determined in the determination process that the frequency of the tracking error signal is lower than the reference signal; detecting the offset of the tracking error signal by using the detection signal and the hold signal; and removing the detected offset from the tracking error signal. By this method, the time of the process of removing the offset from the tracking error signal can be shorten.
A tracking error signal compensation method in accordance with another aspect of the present invention includes the processes of: detecting a track jump of the optical pickup; detecting the offset of the tracking error signal, when the track jump is detected; and removing the detected offset from the tracking error signal. By this method, removal of the offset in the tracking error signal induced can be carried out during the reproducing operation or the recording operation.
The nature, utility, and further feature of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.